The present invention relates to a method of operating a wind turbine. Further, the present invention relates to a controller system for controlling operation of a wind turbine. Finally, the present invention relates to a wind turbine.
Overvoltages (OV) are gaining attention from wind farm owners and wind turbine manufacturers due to the large amount of sensitive power electronics in wind turbines with frequency converters. The transmission system operators are also interested in this phenomenon, since situations in the grid connection system of offshore wind farms have occurred where the insulation systems have been stressed in situations never experienced before. Overvoltages as high as 2 p.u. (per unit) have been observed.
According to the grid code studies, a voltage up to 2 times the nominal grid voltage can be applied at the WTG terminals. For example, in Australia, the temporary OV can be up to 1.6 pu and in Canada-Manitoba the temporary OV can be up to 2.0 pu.
In W. Sweet, “Danish Wind Turbines Take Unfortunate Turn”, IEEE Spectrum, vol. 41, no. 11, pp. 30, 2004, it was reported that on the west coast of Denmark an offshore wind farm called Horns Rev 1 connected by an undersea cable experienced overvoltages as high as 2 p.u. when the main-circuit breaker tripped at the on-land connection point, leaving the wind farm in isolated operation with the cable and the wind farm transformer.
Although such events are rare, this represents a risk of damaging the equipment in the wind turbine. The Danish transmission system operator thus performed investigations of such OV in connection with the planning of new offshore wind farms. These investigations have shown that the OV levels are influenced by many parameters, including operational characteristics of the wind turbines prior to the disconnection, protection systems, control and the accuracy of the representation of the cable and the transformers in the relevant frequency range.
It is desirable to provide a method to handle overvoltage for wind turbines which are highly effective and easy to implement.